The present invention relates to a relief valve. More particularly, the present invention relates to a relief valve for use with a compressed gas storage cylinder (or pressure vessel) which is capable of being activated by either excessive heat or pressure.
In high pressure compressed natural gas systems, it is a requirement to provide a means by which the pressure vessel can be relieved of its gas charge in the event of an excessively high external temperature (e.g., a fire near the vessel). In many instances, however, both pressure and thermal relief are required in one pressure relief system.
In providing for over-pressure relief, the preferred approach is to include a frangible disk (i.e., a disk that ruptures a predetermined pressure) in the system. The disk is exposed directly to the gas charge and acts as a barrier between an interior of the storage cylinder and an escape path within the relief system, which prevents the contents of the storage cylinder from escaping. If the internal pressure of the vessel increases to the disk's burst point, the disk ruptures thereby exposing the escape path for the gas charge.
As for thermal relief, the standard approach has been to incorporate a fusible plug into the design of the system (pressure vessel, valve) that is continuously exposed to the direct pressure of the gas charge. A fusible plug is a fitting that contains a slug of eutectic material that blocks and seals an outlet passage while the external temperature is below a predetermined yield point. When the temperature of the fusible plug reaches the yield temperature, the fusible material melts to provide a pathway for the pressurized gas to escape.
In principle, this approach to thermal relief is acceptable. A problem arises, however, when the fusible plug is exposed to the continuous high pressures of the gas charge at temperatures approaching the yield point of the eutectic material. In such condition, extrusion of the fusible slug may occur, thus producing a potential leak path.
A number of varying strategies can be applied to correct this problem through modification of the fusible plug (e.g., reducing bore diameter, increasing the yield temperature of the eutectic material). None of the strategies mentioned have thus far completely eliminated the problem. In addition, the ability to provide a pressure relief system which is responsive to either excessive temperature or pressure and yet has a reduced design complexity, overall size and manufacturing cost would improve the safety of a wide range of compressed gas storage cylinders.